Communication systems are known to include a communication resource controller, a plurality of communication units, and antennas that transceive a limited number of communication resources. In such communication systems, the communication units communicate with each other via the communication resources, where a communication resource may be an RF channel, a pair of RF channels, a TDM slot, or any medium for carrying RF signals.
To initiate a communication, a communication unit transmits a communication request to the communication resource controller via a communication resource dedicated for control information (control channel). The communication request includes a request for a particular service, such as allocation of a communication resource and identity of a target communication unit(s). For example, the request may be for a group call which identifies all of the communication units in the same group as the initiating communication unit.
Upon receiving this communication request, the communication resource determines whether the communication unit is authorized to access the system and, if so, grants the request. Having granted the request, the communication resource controller allocates a communication resource to the initiating communication unit and the target communication units and then transmits a communication resource allocation message on the control channel. All communication units within the coverage area of the control channel antenna receives the allocation message. However, only the communication units identified in the message, i.e., the initiating and target communication units, will access the allocated communication resource and subsequently partake in the communication.
This method of communication resource allocation works very well in many trunking communication system applications, however, because the coverage area of an antenna can be quite large (typically having a 10 to 30 mile radius), this method has it's limitations. For example, assume that the antenna is centrally located in metropolitan area and the antenna is used by the metropolitan's police force. Further assume that the police force is divided into districts, where each district has its own communication group. With this basic structure, when a police officer initiates a group call for his or her district, all other police officers from that district will receive the group call provided they are within the coverage area of the antenna. If one or more of the receiving police officers are outside of the district, it is impractical for them to receive the call because they are physically unable to respond due to their geographic location, or they may be out of the initiator's jurisdiction. (With the 30 mile coverage radius, a police officer may be up to 60 miles away from the district, making it impossible for him or her to physically respond.) In addition to being unable to physically respond, the police officer's radio is tied up with a communication that he or she can do little about, thus preventing that radio from receiving another call. A call which the officer may be able to respond to based on his or her physical location.
Therefore, a need exists for a method that allows communications to be established based on geographic locations.